Retaining walls - Development length reduction for excess of reinforcement

[Philanthrop60]

ACI 318-19 25.4.10.2 lists the cases where a reduction of development length for excess of reinforcement is prohibited. I'm wondering if any of these conditions is applicable to retaining walls. For example, can Ldh be reduced for Areq / Aprov? It seems that the answer would be "No" if the wall is in a seismic area. What about if the wall is not in a seismic area? Can the hook length Ldh at the base be reduced for the excess of reinforcement? Note that this will affect the footing thickness. Thank you.

 

[DaveAtkins]

I think it can be, although subparagraph (a) gives me pause. It says reduction is prohibited "At noncontinuous supports." To be honest, I have never noticed that exception before. Was it added in more recent versions of ACI?

That being said, I don't think a retaining wall stem is noncontinuous. It has to be continuous with the footing.

DaveAtkins

 

[KootK]

I don't feel that 25.4.10.2 applies and that item (c) below is the code justification for why not. Basically, this isn't a development length problem but, rather, and depending on your perspective:

1) An anchorage problem and;

2) A problem of transferring rebar tension around a closing joint and into the footing toe (continuity).

We discussed this in great length back in 2016 here: Link. That's a marvelous thread but, unfortunately, few are likely to have time to read something of that length in detail.

I believe that the embedment length of the bars can sometimes be less than Ld but determining whether or not that is the case requires one of the following:

1) A strut and tie analysis using curved bar nodes.

2) Pro-rating the rebar efficiency according to the results of testing on concrete closing joints.

3) Creative application of the beam column joint detailing provisions of ACI.

... anything else that is not just development length.

 

[le99]

R25.4.10.2 The excess reinforcement factor (As,required/As,provided), applicable to straight reinforcement is not applicable for hooked or headed bars where force is transferred through a combination of bearing at the hook or head and bond along the bar. Concrete breakout due to bearing at a hook or head was considered in developing the provisions of 25.4.3 and 25.4.4. Because the anchorage strength, and in particular the concrete breakout strength of a hooked or headed bar is a function of the embedment depth to a power slightly more than 1.0 (Shao et al. 2016; Sperry et al. 2017b), a reduction in development length with the application of the excess reinforcement factor could result in a potential concrete breakout failure.

I think the commentary precludes the use of excessive reinforcement factor for reducing ldh of the retaining wall main reinforcement. But, the reduction factor remains applicable to straight bars (at the cost of slab thickness).

 

[JedClampett]

If you have excess reinforcing, can't you use smaller bars with a smaller l_dh?

 

[DaveAtkins]

Subparagraph d clearly prohibits the reduction for hooked bars. This got me to thinking how I could have missed that. I have ACI 318-14, and that exception is not in there! So it previously was allowed (although I don't disagree with KootK that an argument can be made to the contrary).

It seems like every time a new version of a code comes out, new things are added. Hard to keep up

DaveAtkins

 

[le99]

Note, IMO, the reduction remains applicable for hooked bars in beams and slabs. But, watch out for seismic provisions and bars with protential of stress reversal.